Many of the mechanisms of self-renewal and differentiation of hematopoietic precursor cells, including the regulation of these processes, remain to be elucidated. In many respects, further detailed experimental analysis of hematopoiesis depends on the identification, isolation and molecular characterization of pluripotent and committed hematopoietic precursor cells. Immunological identification and study of subsets of cells have led to dramatic progress in understanding other differentiation organ systems; and initial studies reveal impressive heterogeneity of myeloid cell surfaces that should allow manipulation by immunological probes. We propose further development and use of murine and human monoclonal antibodies, specifically directed against small subsets of myeloid cells, to approach the identification and isolation of human hematopoietic precursor cells. Several collectively unique approaches to antibody development and characterization will be employed. Resulting antibodies will be used to isolate precursor cells and to study hematopoiesis in model systems. This work might have eventual broad application for the understanding, diagnosis and treatment of leukemia and aplastic anemia. We have shown that anti-My-1 and other antibodies against mature granulocytes do not react with the colony-forming cells for granulocytes and macrophages (CFC-GM). In contrast, the AHN-7 monoclonal antibody reacts with about 75% of CFC-GM revealing previously undescribed heterogeneity of CFC-GM. We have raised other monoclonal antibodies against myeloid cell lines and characterization of these is progressing. The monoclonal antibody, anti-My-10, reacts with only about 3% of normal human marrow cells; however, this monoclonal antibody binds to all CFC-GM and all terminal deoxynucleotidyl transferase positive cells in marrow. Early results suggest that BFU-E are also My-10 positive. The My-10 antigen is a cell membrane protein of approximately 115 kD. My-11, a cell surface membrane protein of approximately 230 kD, also is expressed by CFC-GM. Further analysis of the cell populations isolated from marrow by these monoclonal antibodies is in progress. We hope this work will yield a detailed antigenic phenotype of myeloid colony forming.